Style pickling is currently broken

[jlstevens]

The pickling of styles is currently not working, presumably since we updated HoloViews to support multiple backends. Note that using the regular pickle module, you can still unpickle your data even if the styles aren't correctly restored.

I also propose adding a few pickle files in the tests directory so we can add some unit tests in order to make sure it doesn't break again without us noticing it.

[jlstevens]

Most of the issues have been addressed in PR #307 (now merged) which involved the style system across backends in general (and not just pickles).

I'll add some unit tests specifically for pickling and then if they pass, I can close this issue.

[jlstevens]

I've added two fairly extensive unit tests (cross backend) in 987f907 and they are passing.

With that I can finally close this issue! I'm very pleased that not only is the bug fixed, but we now have the ability to style the same object across backends. I think this could prove very useful!